Coatings for metal substrates have a variety of applications. Traditionally, coatings have required a multi-coat system, i.e., a primer and topcoat system to combat corrosion. Less costly and faster alternatives are available in the form of Direct to Metal (DTM) coatings, which are self-priming, i.e., act as both a primer and a topcoat. DTM coatings are generally available in semi-gloss, high-gloss, flat or dull matte finishes. High-gloss is understood in the industry as having greater than or equal to 85 gloss units at a 60-degree viewing angle.
A common problem however with high-gloss DTM coating applications is that they do not provide adequate, long-term corrosion protection because they lack corrosion inhibitive pigments. Inhibitive pigments which provide long-term corrosion protection have been unsuitable for incorporation into high gloss DTM coatings, even at low loading levels, because their high oil absorption considerably decreases the gloss or shine of the finish. In high gloss resin coatings, conventional long-term corrosion inhibitive pigments produce an undesirable semi-gloss or flat finish.
Conventional high gloss DTM coatings, such as thin film acrylic and modified acrylic coatings, rely upon their barrier and good adhesion properties and low levels of flash rust inhibitors such as sodium nitrite and ammonium benzoate to provide short-term corrosion protection. Flash rusting causes the appearance of rust spots on a metal substrate shortly after a water-based coating is applied under humid conditions. However, flash rust inhibitors are not sufficiently effective to provide long-term corrosion protection due to their high solubility.
Commercially available petroleum-based corrosion inhibitors with low water solubility, such as metallic sulfonates in non-hazardous air polluting (HAP) solvents in liquid form, pigment compositions containing heavy metal salts (e.g., zinc) of carboxylic acids, and flash rust inhibitors formed with nitrites and nitrates have limited use as long-term corrosion inhibitors in direct to metal applications. However, the high costs and toxicity of these corrosion inhibitors make them cost-prohibitive and impractical for commercial use. Although effective flash rust inhibitors, nitrites and nitrates mixed with amines react to form N-nitrosamines, which are known potent carcinogens. Solvents can evaporate to become hazardous air pollutants (HAPS) and contribute to volatile organic compounds (VOCs). Solvents such as butyl cellusolve (ethylene glycol mono butyl ether) are toxic to humans.
Consequently, a need exists for a corrosion inhibitor that is cost effective, heavy metal free, nitrite free, solvent free, non-toxic, environmentally friendly, does not absorb oil, may be added directly to metal paint coatings, provides long-term corrosion protection, early flash rusting protection, does not adversely affect high gloss resins and may be used solely without other coating components.
In addition to the need for long-term corrosion protection in primer/topcoat and direct to metal applications that would be compatible with high gloss resins, it is also desirable to provide such corrosion resistance in thin films, decreasing the need for multiple applications and the time for film curing.
U.S. Pat. No. 5,043,373 to Hegedus et al. discloses high gloss corrosion resistant coatings that may be applied directly to metal in a single topcoat. The coatings comprise a polyurethane binder and pigment inhibiting system consisting essentially of zinc phosphate, zinc salt of benzoic acids and zinc molybdate. The '373 patent does not disclose or suggest the use of a water-based amine complex salt of a carboxylic acid as a corrosion inhibitor. Further, unlike the present invention, the '373 patent requires the use of heavy metal ions. A drawback to heavy metals is that their high cost and toxicity considerably limits their suitability for commercial use. The present invention, which is in liquid form, is free of heavy metals and does not require synergistic inhibitive pigments such as molybdates and phosphates.
U.S. Pat. No. 5,236,983 to Hegedus et al. discloses polyurethane self-priming topcoats that comprise a polyurethane binder and a combination of inhibitive pigments consisting essentially of an alkaline earth metal phosphosilicate, zinc salts of benzoic acids, an alkaline earth metal phosphate, titanium dioxide, a surface active agent and at least one organic solvent. The surface-active agent of the '983 patent may also be oil soluble. Unlike, the '983 patent, the present invention does not require heavy metal ions. Furthermore, the present invention is not oil soluble, and therefore does not detrimentally affect high gloss resins.
U.S. Pat. No. 4,885,324 by Hegedus et al. discloses a combination primer and topcoat coating for metallic or polymeric composite surfaces comprising a two component aliphatic polyurethane binder and inhibitive pigments consisting essentially of titanium dioxide, zinc molybdate, zinc phosphate and zinc salt of a substituted benzoic acid. Unlike the present invention, the '324 patent relies on heavy metal ions and requires inhibitive pigments such as molybdates and phosphates.
U.S. Pat. No. 5,100,942 to Hegedus et al. discloses high gloss corrosion resistant acrylic coatings comprising an acrylic resin, titanium dioxide, an organic solvent and a combination of corrosion inhibiting pigments consisting essentially of zinc phosphate, zinc molybdate and zinc salt of a benzoic acid. Unlike the '942 patent, the present invention does not require heavy metals or inhibitive pigments. Also unlike the '942 patent, the present invention is a water-based corrosion inhibitor which does not require the addition of any solvents, and may be used alone or combined with conventional paint coating applications.
U.S. Pat. No. 4,243,317 to Grouke et al. discloses corrosion inhibiting aqueous latex paints containing dispersed water soluble complexes of a polyvalent metallic ion, a volatile complexing agent and a corrosion inhibiting anion. Unlike the present invention, the '317 patent utilizes metal ions, which are disadvantageous and undesirable.
U.S. Pat. No. 6,127,467 to Gaglani discloses compositions for inhibiting corrosion in coating applications for metal substrates comprising at least one aminocarboxylate salt containing a metal ion, a pigment, a binder and a solvent. However, the '467 patent does not disclose, teach or suggest the use of a water-based amine complex salt of a carboxylic acid without heavy metal ions, inhibitive pigments or hazardous solvents.
U.S. Pat. No. 4,219,433 to Manabe et al. discloses a metal corrosion inhibitor comprising benzoic acid and/or benzoate, a trialkanolamine, phosphoric acid and/or phosphate and at least one member selected from mercaptobenzothiazole, benzotriazole and tolyltriazole. Unlike the '433 patent, the present invention does not require azoles, which are toxic and therefore hazardous and not environmentally-friendly. The '433 patent teaches a corrosion inhibitor particularly suited for cooling systems and internal combustion engines, and does not disclose an inhibitor suitable for high gloss resins nor multiple applications for metal substrates, unlike the thin film water-based coatings of the present invention.
U.S. Pat. No. 4,051,066 to Miksic et al. discloses solid corrosion inhibiting elastomers containing an elastomeric matrix containing a corrosion inhibiting amount of cyclohexamethyleneimine dinitrobenzoate. The '066 patent teaches that corrosion inhibitors must be milled, entrapped or uniformly dispersed into rubber-like polymers or elastomers as volatile corrosion inhibitors that must have sufficient vapor pressure. Unlike the '066 patent, the present invention does not require that it be milled. The '066 patent disclosure does not teach or suggest a stable liquid form corrosion inhibitor as in the present invention, which is not volatile and may be applied alone or easily added to conventional paint coating formulas.
U.S. Pat. No. 5,519,074 to Kramer et al. discloses crystalline complexes of morpholine derivatives with keto-acids as corrosion inhibitors. Unlike the present invention, the '074 patent requires the use of keto-acids and a reaction calling for two parts keto-acid to one part base, unlike present invention. Further, the present invention may be applied to a metal substrate without the addition of a coating composition, unlike the '074 patent. The present invention also forms a liquid product, whereas the '074 patent produces a crystalline solid product. The solid compound corrosion inhibitor of the '074 patent must be diluted in water and grinded into a paint coating, unlike the present invention. In addition, unlike the present invention, the '074 patent teaches that washing, filtration, drying and the use of environmentally unfriendly protic or aprotic solvents, resulting in volatile organic compounds (VOCs), are required to isolate the '074 invention. The present invention does not require washing, drying, filtration or the use of solvents. In addition, the present invention, demonstrates significantly enhanced barrier properties when compared to the commercial embodiment of the '074 invention, namely Irgacor® 1405.
The present invention overcomes the disadvantages and/or shortcomings of known prior art metal substrate corrosion inhibitors and provides a significant improvement thereover.